High-temperature Tests Research Articles

High-Temperature Mechanical Characterization of Materials for Extreme Environments

The growth of advanced technologies involves the development of materials that can withstand extreme environmental conditions, particularly elevated temperatures. This paper presents an in-depth examination of the mechanical properties of materials designed specifically for use in high-temperature environments, such as however confined to aviation, nuclear-powered reactors, and electrical power systems. Relevant significance is associated with assessing the mechanical robustness, resilience to deformation under constant stress, and ability to cope with high temperatures over a longer time for these materials. This study explores recent developments in materials science, focusing on the products made in alloys, ceramics, and composite materials such as nickel-based superalloys, silicon carbide (SiC), and composite based on zirconium diboride (ZrB2). A significant focus is placed on innovative testing methods, including high-temperature tensile tests, thermal shock resistance assessment, and fatigue testing, as these play a critical role in evaluating the performance of substances under challenging conditions. Further, this study explores the consequences of these findings on the choice of materials and the design process in engineering applications. Titanium superalloy operates effectively at lower temperatures, whereas Nickel-based 70% of the initial strength when heated to a higher temperature of 1100°C superalloy behaves superior under more extreme conditions.

Experimental study of CO2/O2 corrosion on pipelines during carbon capture, utilization and storage process

In this paper, high-temperature and high-pressure corrosion test was used to simulate the corrosion process of multi-component thermal fluid (CO2/O2/H2O) on oil pipes (N80 steel), casings (J55, TP90H, P110 steel, and transportation pipelines (20# steel) in the process of CCUS technology application. The results show that during the entire corrosion process, the oil layer casing and surface casing are corroded most seriously in this environment, and the corrosion forms of these five steel materials in the CO2/O2 environment are uniform corrosion and local corrosion, and the corrosion products mainly consist of FeCO3, Fe2O3, Fe3O4 and FeOOH crystals. These oxides form a double-layer film structure on the surface of the steel. The inner layer is denser and consists mostly of FeCO3 crystals, which has better protection against further corrosion of the internal steel. The outer layer is loose and porous, and its components are mostly FeCO3 crystals. Fe2O3, FeOOH, Fe3O4 and FeCO3 crystals, have poor corrosion protection for internal steel. Due to the difference in corrosion products of the inner and outer film layers, relatively obvious local corrosion appeared on the surfaces of the 5 steel materials.

Influence of post-weld heat treatment on welding residual stress in U-rib-to-deck joint

Welding residual stress (WRS) significantly influences the fatigue life and safety of welded steel structures. Post-weld heat treatment (PWHT) is a common method employed to mitigate WRS. In present study, the WRS evolution law and relaxation mechanism of U-rib-to-deck joints was investigated during PWHT. High-temperature heat treatment tests with varying holding times were conducted on U-rib-to-deck joints to reduce WRS. The WRS distributions were measured using the hole-drilling method before and after PWHT. ABAQUS software, incorporating the Norton-Bailey creep constitutive model, was employed to simulate the WRS evaluation during both welding and PWHT processes. The simulated values were validated against the measured data, and the effects of different holding times, holding temperatures, and heating and cooling rates during PWHT on WRS relaxation were discussed. The results demonstrate that PWHT effectively reduces WRS and leads to a more uniform distribution. Extending the PWHT holding time from 0.5 h to 1 h and 2 h results in a gradual decrease in WRS relaxation at 600 °C, primarily occurring in the high-value stress region of the weld zone with a small amplitude. Elevating the PWHT temperature from 500 °C to 550 °C and subsequently to 600 °C amplifies the significance of WRS relaxation. Variations in heating and cooling rates (200 °C/h and 80 °C/h, 100 °C/h and 40 °C/h, 50 °C/h and 20 °C/h) lead to a gradual decrease in WRS relaxation across the entire model. Furthermore, considering the creep effect, the stress relaxation rate in the weld is approximately 70 %–80 %, which found to be about 2–3 times higher than that without considering the creep effect. By understanding the evolution law and relaxation mechanism, this study provides valuable insights for designing PWHT processes and assessing their impact on structural safety and fatigue life.

Mechanical performance of a bond-type anchorage system for CFRP cable exposed to elevated temperature

Both high-temperature pullout tests without and with pre-tension load were conducted on the bond-type anchorage system specimens with bonding medium of UHPC (Ultra-high Performance Concrete) for Carbon fiber reinforced polymer (CFRP) cable in order to fully understand the bonding behavior of CFRP cable-UHPC interface under high temperature. The variation laws of ultimate pullout load, average bond strength of CFRP cable-UHPC interface and slippage corresponding to ultimate pullout load were uncovered in two types of high-temperature pullout tests. Then, the bond performances of CFRP cable-UHPC interface in high-temperature pullout tests with pre-tension load ratio from 0.2 to 0.6 and target temperature in the range of 100–210 ℃ were compared to these in high-temperature pullout tests without pre-tension load in order to identify the effect of temperature-load path. Finally, theoretical models for determining average bond strength of CFRP cable-UHPC interface and critical bond length of CFRP cable for the bond-type anchorage system under high temperature were developed by incorporating effects of high temperature and pre-tension load ratio. The obtained results demonstrate that the dominant failure mode of the bond-type anchorage system in high-temperature pullout tests is slip failure of CFRP cable-UHPC interface, and shear failure morphologies of embossed rid for CFRP tendon are closely correlated with target temperature. In high-temperature pullout tests without pre-tension load, the average bond strength of CFRP tendon-UHPC interface gradually decreased as the increasing target temperature, the corresponding attenuation ratios in temperature ranges of 25–300 ℃ and 300–400 ℃ are 84% and 47%, respectively. In high-temperature pullout tests with pre-tension load, the average bond strength of CFRP tendon-UHPC interface approximately linearly decreased as the increasing target temperature or pre-tension load ratio. The presence of pre-tension load results in acceleration of damage of CFRP cable-UHPC interface, and the damage degree is proportional to pre-tension load. The developed theoretical models of mechanical properties for the bond-type anchorage system exposed to high temperature can accurately predict average bond strength of CFRP cable-UHPC interface and critical bond length of CFRP cable.

Near-net-size porous mullite ceramics with tuneable sintering shrinkage for thermal insulation applications

The microstructure and properties of porous ceramics are severely affected by volume shrinkage during the sintering process. Therefore, reducing sintering shrinkage while maintaining the properties of porous ceramics is a serious challenge. Herein, near-net-size porous mullite ceramics with tuneable sintering shrinkage were prepared by the foam-gelcasting method using kyanite as raw material and aluminum fluoride and aluminum powders as additives through the volume expansion generated by the sintering reaction. The effects of Al powder content on the sintering shrinkage, porosity, compressive strength and thermal conductivity of porous mullite ceramics were investigated. The results show that the sintering shrinkage gradually decreases with increasing Al powder content, and expansion is even produced. When the Al powder content was 5 wt%, the sintering shrinkage of porous mullite ceramics was −0.21 %, with porosity of 84.2 %, compressive strength of 2.45 ± 0.15 MPa, and thermal conductivity of 0.19 W/(m·K) at room temperature. The results of the high-temperature fire resistance test showed that the temperature difference between the front and back sides of the sample under the flame gun reached 800 °C, indicating good thermal insulation performance. The near-net-size sintering process of porous ceramics can effectively reduce the subsequent processing cost, open new paths for their production, and broaden the application fields of porous mullite ceramics.

Steam oxidation properties of graphene reinforced bioinspired laminated CoCrFeNiMn high-entropy alloy matrix composites at 1000 ℃

Laminated CoCrFeNiMn high-entropy alloy (HEA) matrix composites reinforced with 1.0 wt% graphene nanoplatelets (GNPs) were fabricated by mechanical ball milling and flake powder metallurgy, and were isothermal oxidized at 1000 ℃ for 100 h. Vacuum hot-pressure sintering (VHPS) shows a distinct nacre structure with the microstructure composed of FCC matrix phase, Cr23C6, CrMn1.5O4 precipitated phases, numerous dislocations and twins. High-temperature oxidation tests showed that the composites had mass gains at 12 h intervals from 12 to 100 h respectively. The oxidation kinetic curve changes from a linear pattern in the early stages to an exponential pattern in the later stages, which demonstrates better long-term oxidation resistance. The anisotropy of the laminated structure results in excellent resistance to high-temperature steam oxidation in the vertical lamellar direction. Observation of the cross-section reveals that although Cr2O3 (inner layer) is present, (Mn, Cr)3O4 and Mn3O4 are the dominant oxides (outer layer). Elemental depletion zones for Mn and Cr exist in the region of the matrix near the oxide scales. The results show that the oxidation resistance of the laminated GNPs/CoCrFeNiMn composites is mainly influenced by the diffusion of Mn and Cr elements, the microstructure of the laminations and the internal oxidation.

On the applicability of carbon steels K55 and L80 for underground hydrogen storage

To evaluate the possibility of hydrogen storage in depleted gas reservoirs, natural gas storage facilities, aquifers and salt caverns, the applicability of ferritic pearlitic K55 and tempered martensitic L80, both very frequently used as casings and tubings, has been investigated. Materials were investigated by means of high-pressure, high-temperature autoclave tests and analyses of the hydrogen uptake. The autoclave tests were performed on tensile specimens loaded with a spring at 90 % of the specified minimum yield strength, additionally the samples were analysed to determine the hydrogen uptake. Different gas compositions were considered (pure hydrogen, with or without the presence of CO2/H2S) under a hydrogen partial pressure of 120 bar. The tests were conducted in dry or wet environments. From the results, it can be seen that the hydrogen uptake is low even under the most severe conditions. However, from the mechanical test conducted in this study, it appears that the ferritic pearlitic K55 steel seems to be a suitable pipe material for underground hydrogen storage, and the higher strength steel L80 steel can be used only in non-sour environments (no significant amount of H2S in the reservoir, which is a priori the case of underground storages).

Investigation on micro region deformation of residual indents during temperature recovery

Observation of residual indent is widely adopted to evaluate the mechanical properties of material in temperature indentation test. However, the potential influence of the difference between the testing and observing temperatures on the material is always neglected. In this study, crystal plasticity finite element model of copper was constructed, and the processes of high-temperature indentation test and specimen temperature reduction to the observed temperature after the test were simulated. The simulations showed that the error of test hardness can reach up to 4.92% as the residual indent area decreases with temperature decreasing from 800 °C to 20 °C. The stress values of the indentation influence zone were also reduced considerably, in which the maximum stress at 20 °C was 59% of that at 800 °C. The distribution of stresses at surface and subsurface both changed substantially, while the depth of the influence zone increased by 31%. The mechanical properties of material were also found to affect the micro region deformation of residual indents during temperature recovery. Therefore, the influence of temperature variation between the testing and observing processes on residual indent should be seriously considered.

Investigation of High-Temperature Oxidation of Homogeneous and Gradient Ni-Cr-Al Coatings Obtained by Detonation Spraying

The high-temperature oxidation of homogeneous and gradient coatings based on Ni-Cr-Al obtained by detonation spraying is investigated. To assess the resistance to high-temperature oxidation of Ni-Cr-Al coatings, cyclic tests were carried out at a temperature of 1000 °C for 50 cycles. The assessment of high-temperature oxidizing ability was carried out by measuring the weight gain of samples after each cycle. After high-temperature oxidation tests, the morphology and chemical composition of the coating structure in the cross-section were investigated using SEM/EDS. The phase composition of the samples was studied by X-ray diffraction (XRD) phase analysis. Visual analysis of the sample surface under study after high-temperature oxidation showed that the surface of homogeneous or gradient Ni-Cr-Al coatings remained undamaged. The results of X-ray phase analysis showed the peaks of Al2O3 in Ni-Cr-Al gradient coatings are more expressed and intense compared to homogeneous coatings of Ni-Cr-Al. Gradient coatings also retain an increased chromium content compared to homogeneous Ni-Cr-Al coatings. This increased chromium content can slow down mixing or diffusion between different phases of the material at their boundary, which, in turn, contributes to increasing the resistance of the gradient coating to oxidation.

Fracture behavior of AZ31B magnesium alloy under high-temperature ultrasonic-assisted shearing with related ductile-brittle transformation mechanism

A wide width is one of the fundamental requirements for magnesium alloy sheets to fulfill the lightweight needs of the aerospace and rail transportation industries. Nevertheless, ensuring the dimensional integrity of wide-format magnesium alloy sheets is a challenge in most manufacturing facilities. Simultaneously, processing operations are susceptible to encountering production issues, including but not limited to significant loss of raw materials, elevated costs, environmental contamination, and safety risks. Therefore, to fulfill the operational demands of the hot rolling finishing line for wide sheets made of magnesium alloy, the present study employed ultrasonic-assisted shearing (UAS) to carry out a high-temperature shearing test investigation on the AZ31B magnesium alloy sheet. In this research, we employed scanning electron microscopy, optical microscopy, and a micro-hardness tester to investigate the relationship between ultrasonic amplitude and fracture behavior, deformation mechanism, and mechanical properties of magnesium alloy sheets following exposure to ultrasonic vibration. The fracture morphology of the sheet fracture demonstrates that the behavior of the magnesium alloy sheet fractures under ultrasonic vibration changes from ductile fracture under conventional shearing to brittle fracture under ultrasonic-assisted shearing. The primary cause of this transformation between ductile and brittle is the effect of ultrasonic vibration on the material's internal energy. High-energy ultrasonic amplitude action causes magnesium to absorb a significant amount of ultrasonic energy. This results in an increase in the number of dynamic re-crystallization grains in the material organization, a decrease in the number of twins, and significant refinement of the grains. The sheet's hardness also changes along the rolling and normal directions, with a uniform distribution. The magnesium alloy sheet's fracture quality is optimal when the ultrasonic amplitude is 10 μm in the UAS process. This treatment results in notable enhancements in the section's straightness and brightness. Our study aims to advance the mass manufacturing of wide-format magnesium alloy sheets by investigating a cost-effective and high-yield technique for shearing magnesium alloy sheets.

Conformal Fabrication of Thick Film Platinum Strain Gauge Via Error Regulation Strategies for In Situ High-Temperature Strain Detection.

Monitoring high-temperature strain on curved components in harsh environments is a challenge for a wide range of applications, including in aircraft engines, gas turbines, and hypersonic vehicles. Although there are significant improvements in the preparation of high-temperature piezoresistive film on planar surfaces using 3D printing methods, there are still difficulties with poor surface compatibility and high-temperature strain testing on curved surfaces. Herein, a conformal direct ink writing (CDIW) system coupled with an error feedback regulation strategy was used to fabricate high-precision, thick films on curved surfaces. This strategy enabled the maximum amount of error in the distance between the needle and the substrate on a curved surface to be regulated from 155 to 4 μm. A conformal Pt thick-film strain gauge (CPTFSG) with a room-temperature strain coefficient of 1.7 was created on a curved metallic substrate for the first time. The resistance drift rate at 800 °C for 1 h was 1.1%, which demonstrated the excellent stability and oxidation resistance of the CPTFSG. High-temperature dynamic strain tests up to 769 °C revealed that the sensor had excellent high-temperature strain test performance. Furthermore, the CPTFSG was conformally deposited on an aero-engine turbine blade to perform in situ tensile and compressive strain testing at room temperature. High-temperature strain tests were conducted at 100 and 200 °C for 600 and 580 με, respectively, demonstrating a high steady-state response consistent with the commercial high-temperature strain transducer. In addition, steady-state strain tests at high temperatures up to 496 °C were tested. The CDIW error modulation strategy provides a highly promising approach for the high-precision fabrication of Pt thick films on complex surfaces and driving in situ sensing of high-temperature parameters on curved components toward practical applications.

High-temperature tribological behavior of nanolayered TiAlN/TiSiN coating deposited on WC-Co cemented carbide

In this study, high-temperature tribological behavior of nanolayered TiAlN/TiSiN coating was evaluated against the Al2O3 counter-body using a pin-on-disk tribometer. The coating was deposited on the WC-Co substrate, in an industrial unbalanced magnetron sputtering system. Tribological tests were conducted at room temperature, 500, 600, 700, and 760 °C, in air and nitrogen atmospheres. After the tests, the coating was examined using confocal microscopy, tactile profilometry, scanning electron microscopy, focused ion beam, energy dispersive spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The coating retained its microstructure and mechanical properties after exposure to high temperatures. At lower temperatures coating exhibited abrasive and adhesive wear mechanisms, while at higher temperatures abrasive and oxidative wear mechanisms were observed. In high-temperature tests, AlO, TiO and SiO were detected inside of wear tracks, in both atmospheres. However, the oxide thickness was significantly lower in tests with nitrogen atmosphere. Additionally, the top of the oxide layer was enriched in AlO with respect to TiO and SiO. The enrichment of AlO was more pronounced in nitrogen atmosphere. The coating tested in air exhibited slightly higher and more unstable coefficient of friction (COF) values, than in nitrogen atmosphere. This is attributed to the increased oxidation in air atmosphere. At room temperature tests in nitrogen, the wear rate was approximately 3 times lower than in air. At higher temperatures the wear rate was lower than at room temperature, in both atmospheres, due to formation of protective oxides. However, with the increase in testing temperature the wear rate increased. The reason for such behavior is the loss of substrate's and coating's hardness at high temperatures, and thickening of the oxide layer which accelerated its removal. The latter effect is not so pronounced in nitrogen atmosphere due to thinner oxide layer, which agrees with the lower wear rate in nitrogen atmosphere at high temperatures than in air.

Tribological, oxidation and corrosion properties of ceramic coating on AISI H13 steel by rare earth-Cr composite boronizing

In this paper, a rare earth-Cr composite boronizing method to prepare boride ceramic coatings for AISI H13 steel was proposed. The effect of rare earth elements and Cr on the microstructure, hardness, wear resistance, high temperature oxidation resistance, and corrosion of boride ceramic layers was researched. The characteristics of the boride layers were investigated by means of X-ray diffraction (XRD), scanning electron microscope (SEM), electron probe microanalysis (EPMA), wear tests, high-temperature oxidation tests, and corrosion immersion tests, etc. The results showed that the boride layer comprised of Fe2B, FeB, CrB and Cr2B phases. The addition of rare earth CeO2 and Cr nanopowder promoted the growth of the boride layer, and its surface hardness increased by 3.5 times compared with that of tempered AISI H13 steel. In addition, the addition of CeO2 and Cr significantly reduced the wear rate of the boride layer, with the surface wear mechanism being abrasive oxidative wear. Furthermore, the oxidation rate of the boride layer was 71 % lower than that of AISI H13 steel, demonstrating excellent high-temperature oxidation resistance; and the boride layer showed good acid corrosion resistance in corrosion immersion tests.

High-Temperature Bending Tests of Reaction-Sintered Silicon Carbide-Based Ceramic Materials

The high-temperature strength properties of silicon carbide-based ceramic materials reaction-sintered in air and vacuum are proved by experimental data. The ultimate bending strength of pressed and cast ceramic material samples at temperatures up to 1400°C was measured as a criterion. High-temperature tests demonstrate the guaranteed strength of ceramics above 100 MPa. An example of the manufacture of a complex shaped ceramic product using the hot slip casting processing under pressure into additive water-soluble forms is given. An approval of the combined technology makes it possible to manufacture bladed ceramic elements with a thickness of about 1 mm.

Effects of Composition Variations on Mechanochemically Synthesized Lithium Metazirconate-Based Ceramics and Their Resistance to External Influences

The study examines the influence of variations in the compositions of components for the production of lithium-containing ceramics based on lithium metazirconate obtained by the method of mechanochemical grinding and subsequent thermal sintering. For component variation, two compositions were used, consisting of zirconium dioxide (ZrO2) and two distinct types of lithium-containing materials: lithium perchlorate (LiClO4·3H2O) and lithium carbonate (Li2CO3). Adjusting the concentration of these components allowed for the production of two-phase ceramics with varying levels of impurity phases. Using X-ray phase analysis methods, it was determined that the use of LiClO4·3H2O results in the formation of a monoclinic phase, Li2ZrO3, with impurity inclusions in the orthorhombic phase, LiO2. On the other hand, when Li2CO3 is used, the resulting ceramics comprise a mixture of two phases, Li2ZrO3 and Li6Zr2O7. During the studies, it was established that the formation of impurity inclusions in the composition of ceramics leads to an increase in the stability of strength properties with varying mechanical test conditions, as well as stabilization of thermophysical parameters and a decrease in thermal expansion during long-term high-temperature tests. It has been established that in the case of two-phase ceramics Li2ZrO3/Li6Zr2O7 in which the dominance of the Li6Zr2O7 phase is observed during high-temperature mechanical tests, a more pronounced decrease in resistance to cracking is observed, due to thermal expansion of the crystal lattice.

Reliability evaluation of thick Ag wire bonding on Ni pad for power devices

In recent years, silicon carbide (SiC) semiconductors have gained traction in high-performance power devices due to their ability to operate at high temperatures (above 200 °C). However, traditional interconnection materials are not sufficiently reliable at such elevated temperatures. Typically, in power devices, thick Al wires with diameters of 200 μm or larger are commonly used to bond Al electrodes of semiconductor chips to their corresponding Cu leadframes or substrates. Nevertheless, the low melting point of Al can lead to recrystallization at high temperatures, thereby raising concerns about reliability and a decrease in wire strength and fatigue properties. Although thick Cu wires with higher melting points have been explored as potential solutions, their high hardness and susceptibility to work hardening during bonding can damage the chip beneath the bonding area, making them impractical. In response to these challenges, we investigated the use of thick Ag wires. However, direct bonding of thick Ag wires to Al pads can form intermetallic compounds (IMC) in the bonding area, leading to concerns about decreased reliability. To address this issue, we experimented with a bonding method that utilized thick Ag wires on Ni pads. In this study, we evaluated the reliability of thick Ag wire bonding under various stress conditions, including high-temperature storage tests (HTST) and temperature cycling tests (TCT). Our results showed that thick Ag wire bonding on Ni pads exhibited high bond strength and prevented IMC formation after HTST. Nonetheless, bond failure of the Ag wires could occur after TCT due to the mismatch in the coefficient of thermal expansion (CTE) between the Ag wire and SiC chip, particularly under extremely severe TCT conditions. We found this issue could be mitigated by introducing a soft metal layer, such as Al, between the Ni pad and the SiC chip. This reduced internal stress and enabled the device to pass TCT under harsher conditions with reliable performance.

Analysis of forced convection in the HTTU experiment using numerical codes

The High Temperature Test Unit (HTTU) was an experimental set-up to conduct separate and integral effects tests of the Pebble Bed Modular Reactor (PBMR) core. The annular core consisted of a randomly packed bed of uniform spheres. Natural convection tests using both nitrogen and helium, and forced convection tests using nitrogen, were conducted. The maximum material temperature achieved during forced convection testing was 1200 °C. This paper presents the numerical analysis of the flow and temperature distribution for a forced convection test using 3D CFD as well as a 1D systems-CFD computer code. Several modelling approaches are possible, ranging from a fully explicit to a semi-implicit method that relies on correlations of their associated phenomena. For the comparison between codes, the analysis was performed using a porous media approach, where the conduction and radiative heat transfer were lumped together as an effective thermal conductivity and the convective heat transfer was correlated between the solid and gas phases. The results from both codes were validated against the experimental measurements. Favourable results were obtained, in particular by the systems-CFD code with minimal computational and time requirements.

Study on physical property regulation and high-temperature heat storage properties of Al–Si composite phase change materials prepared by one-step pressing

In this paper, we investigate a metal composite prepared using a one-step pressing method, where Al–Si alloy acts as the phase change materials (PCMs) and Al2O3/AlN serve as the structural support materials. The alloy formation and PCM composite preparation are synthesized in a single step. The study aims to characterize the microstructure, thermodynamic, and thermal properties of the composite material. To assess the heat storage properties of the prepared composite PCMs at temperatures higher than 600 °C, a high-temperature test system is established using a high-frequency induction heater. Additionally, we explore how to improve the issue of liquid leakage under high-temperature conditions. The results demonstrate that Al2O3 and AlN (ceramic materials) exhibit excellent chemical compatibility with Al–Si composite PCMs. After sintering, the ceramic materials form a strong bond with Al–Si, effectively retaining the liquid phase of Al–Si alloy within the composite material. Consequently, the Al–Si composite PCMs do not exhibit high-temperature liquid leakage under elevated conditions. Both Al2O3/Al–Si and AlN/Al–Si composites possess favorable high temperature heat storage properties. The phase change enthalpy of Al2O3/Al–Si composite reaches 400 kJ/kg, with a thermal conductivity of 36 W/(m·K). Similarly, the AlN/Al–Si composite achieves a phase change enthalpy of 404 kJ/kg and a thermal conductivity of 43 W/(m·K). Both composites fulfill the requirements for high-temperature heat storage applications.

Creep and stress relaxation experiments of 1960-grade high-strength steel wire at elevated temperatures

With the increasing spacing of large-span structures, the application of ultra-high-strength steel wire with a strength approaching or exceeding 2000 MPa is rapidly expanding. However, the related research on its creep and stress relaxation behaviour is still limited. In this paper, a total of 30 high-temperature creep tests and 31 high-temperature stress relaxation tests were conducted on 1960-grade high-strength steel wire (1960-HSSW) with a diameter of 5.0 mm. The full process creep strain curves were obtained by reinserting and resetting the high-temperature extensometer. The maximum creep strain measured was 50.67 %. The creep and stress relaxation curves of different high-strength steel wires (steel strands) were compared, revealing significant variations in mechanical properties among different steel wires. The steady-state creep rate and high-temperature creep rupture critical stress ratio of the 1960-HSSW were given. Especially, the critical stress ratio became lower than the yield strength reduction coefficient at temperatures exceeding 400 °C and it indicated that the 1960-HSSW could undergo creep rupture before yield. The stress relaxation coefficient was introduced to consider the stress relaxation of 1960-HSSW. Stress relaxation curves for the 1960-HSSW were provided at temperatures ranging from 100 to 550 °C. Moreover, based on the microstructure observed under transmission electron microscopy, it was found that damage occurred in the 1960-HSSW after exposure to 300 °C. Therefore, a critical temperature of 300 °C was recommended for the fire protection of 1960-HSSW, which could be the theoretical support for its fire protection design. Additionally, a high-temperature creep prediction model, which includes expressions for the termination time, was established for the 1960-HSSW. As a result, a conservative prediction of the high-temperature creep for the 1960-HSSW was provided. Moreover, A stress relaxation coefficient model was also provided. The predicted results of the aforementioned models were in good agreement with experimental results.

Elevated temperature properties of foam concrete: Experimental study, numerical simulation, and theoretical analysis

Foam concrete, as an inorganic cellular material possessing exceptional thermal insulation and fire resistance properties, emerges as a promising candidate for thermal insulation. To achieve a more precise evaluation of its performance under elevated temperatures, the present study comprehensively investigates the thermal characteristics of foam concrete with a combined experimental, numerical, and theoretical approach. Firstly, a high-temperature test is conducted to investigate the influence of temperature and foam concrete density on its macroscopic appearance and internal temperature distribution. Meanwhile, the thermal conductivity of foam concrete at various densities and temperatures is directly measured with a thermal conductivity analyzer. Subsequently, to acquire comprehensive specific heat and thermal conductivity parameters of foam concrete, with a numerical model established within ABAQUS, the genetic algorithm is applied to identify these thermal parameters based on the measurement data from the elevated temperature test. In addition, CT scanning and 3D reconstruction of foam concrete before and after exposure to elevated temperatures are utilized to extract pore characteristics. This facilitates the thermal conductivity calculation using theoretical models, enabling the determination of the thermal conductivity of foam concrete above 600 °C. Through a comparison of the experimental, numerical, and theoretical results, it is recommended to consider the variation of specific heat in three dehydration stages due to the observed notable dehydration. Due to the effect of cracks, the numerically determined thermal conductivity is slightly higher compared to the experimentally and theoretically determined thermal conductivity. Based on the analysis, an improved formula predicting the specific heat of foam concrete, and a formula determining the thermal conductivity of foam concrete subjected to elevated temperature are proposed.